Segfault in fix_type_field

Imported from http://bugzilla.roxen.com/bugzilla/show_bug.cgi?id=4362

Reported by Martin Stjernholm mast@roxen.com

The following program causes a segfault after reporting the compilation error:

  int main()
  {
    array m;
    foreach (({"Regexp.PCRE.Widestring"}), string symbol)
      catch {
	if (!zero_type(all_constants()[sym]) ||
	    !zero_type(master()->resolv(symbol)))
	  m += ({symbol});
      };
  }

(gdb) bt
#0  fix_type_field (n=0xa10ac0) at
/home/mast/Pike/no-changes-7.7/src/las.c:3893
#1  0x0000000000508955 in optimize (n=0xa10ac0)
    at /home/mast/Pike/no-changes-7.7/src/las.c:5530
#2  0x000000000050d776 in dooptcode (name=0x960330, n=0xffffffff,
type=0xa10ac0,
    modifiers=9781792) at /home/mast/Pike/no-changes-7.7/src/las.c:5924
#3  0x000000000042d67f in yyparse () at language.yacc:948
#4  0x00000000004ce072 in run_pass2 (c=0xa27390)
    at /home/mast/Pike/no-changes-7.7/src/program.c:6831
#5  0x00000000004ce2fb in compile (aprog=0x0, ahandler=<value optimized
out>, amajor=-1,
    aminor=-1, atarget=0x987598, aplaceholder=0x0)
    at /home/mast/Pike/no-changes-7.7/src/program.c:7045
#6  0x0000000000517bb7 in f_compile (args=6)
    at /home/mast/Pike/no-changes-7.7/src/builtin_functions.c:4075
#7  0x000000000043706f in eval_instruction (pc=<value optimized out>)
    at /home/mast/Pike/no-changes-7.7/src/interpret_functions.h:2284
#8  0x000000000043a85b in catching_eval_instruction (
    pc=0xa1a87b "!1\026\002&#65533;)K\anF\b[\f")
    at /home/mast/Pike/no-changes-7.7/src/interpret.c:2223
#9  0x0000000000439a4f in eval_instruction (pc=<value optimized out>)
    at /home/mast/Pike/no-changes-7.7/src/interpret_functions.h:1273
#10 0x000000000043a91f in mega_apply (type=<value optimized out>,
    args=<value optimized out>, arg1=<value optimized out>, arg2=<value
optimized out>)
    at /home/mast/Pike/no-changes-7.7/src/interpret.c:2191
#11 0x00000000004be4fe in o_cast (type=0x952e70, run_time_type=5)
    at /home/mast/Pike/no-changes-7.7/src/operators.c:667
#12 0x00000000004bf646 in f_cast () at
/home/mast/Pike/no-changes-7.7/src/operators.c:887
#13 0x0000000000435f7c in eval_instruction (pc=0xa1b776 "K\tnF\n[e")
    at /home/mast/Pike/no-changes-7.7/src/interpret_functions.h:1882
#14 0x000000000043a85b in catching_eval_instruction (pc=0xa1b76f
"\031R'\002\237")
    at /home/mast/Pike/no-changes-7.7/src/interpret.c:2223
#15 0x0000000000439a4f in eval_instruction (pc=<value optimized out>)
    at /home/mast/Pike/no-changes-7.7/src/interpret_functions.h:1273
#16 0x000000000043a91f in mega_apply (type=<value optimized out>,
    args=<value optimized out>, arg1=<value optimized out>, arg2=<value
optimized out>)
    at /home/mast/Pike/no-changes-7.7/src/interpret.c:2191
#17 0x0000000000425665 in main (argc=4, argv=0x7ffffc8a4d18)
    at /home/mast/Pike/no-changes-7.7/src/main.c:569